Facile synthesis of soybean protein-based phosphorus-nitrogen flame retardant for poly(lactic acid)

An eco-friendly P-N-containing flame retardant, soybean protein-based methylphosphonate (SPDP), was synthesized based on soybean protein for PLA by Mannich reaction. The grafting of dimethyl phosphite onto soybean protein molecule by the short reaction process, improved the flame retardant efficiency of protein-based flame retardants significantly, and the high-efficiency flame-retardant PLA/SPDP composites were successfully prepared. The addition of 7wt.% SPDP increased the limited oxygen index (LOI) value of PLA from 19.4% to 26.3%, and achieved V0 rating in the UL-94 vertical burning tests. And the heat release rate (HRR) and total heat release (THR) of PLA/SPDP composites decreased observably. The results of TGA, TG-IR, FTIR, XPS and SEM indicated that SPDP increased the condensed components by producing P-containing composites in condensed phase and decreased the flammable gaseous compounds releasing the un-flammable NH3, resulting the positive effect on char formation of PLA.